Vacuum needed to properly run a fpr?

Ok, does anyone know who much vacuum it takes to make a fuel pressure regulator work properly? I'm seeing 12in Hg at idle and my fuel pressure dosen't change like is should when I remove the vac line. It's 38psi at idle, it does go up when driving to around 42psi. This is with the stock regulator, I tried an adjustable one with the same results, I'm also running a 255lph high pressure pump as I have a dry nitrous kit on the car.
Thanks for any help.
Chuck

Thanks for the response, the only reason I asked was because when the car was mostly stock the fuel pressure at idle was around 32psi and would go up to around 40psi when at WOT. I changed the cam and fuel pump at the same time and now the fuel pressure dosen't act like it used to. I'm just trying to figure out if it's the lack of vacuum or the high pressure pump that is responsible for the change in fuel pressure.
Thanks again,
Chuck